WEEK 10 (Chromatin structure and nucleosomes)

Question 1

What are Nucleosomes?

Answer 1

a structural unit of a eukaryotic chromosome, consisting of a length of DNA coiled around a core of histones

Question 2

What does a single Nucleosome contain

Answer 2

8 histone molecules + 146 base pairs of DNA

Question 3

What are the 4 different types of histone molecules?

Answer 3

• H2A
• H2B
• H3
• H4

Question 4

Describe the Chromatin structure

Answer 4

• DNA exists in the condensed, chromatin form to fit into the nucleus
• DNA loops twice around a histone octamer to form a nucleosome (“beads on a string”)
• H1 binds to the nucleosome and to “linker DNA” stabilising the chromatin fiber
• Phosphate groups give DNA a -ve charge; LYSINE and ARGININE give histones a +ve charge

Question 5

What are the properties of DNA in mitosis?

Answer 5

• In mitosis, DNA condenses to form chromosomes
• DNA and histone synthesis occurs during the S phase

Question 6

Describe mitochondrial DNA

Answer 6

• circular
• does not utilise histones

Question 7

What is the difference between Heterochromatin and Euchromatin?

Answer 7

Heterochromatin is HIGHLY CONDENSED and appears darker on the electron microscope (e.g Barr bodies). It is Sterically inaccessible, TRANSCRIPTIONALLY INACTIVE, increased methylation and decreased acetylation.

Euchromatin is LESS CONDENSED, appears lighter on the electron microscope, TRANSCRIPTIONALLY ACTIVE and sterically accessible.